The US-led coalition has failed to prove that Iraq posed a "manifest and imminent" threat that justified war, former chief UN weapons inspector Hans Blix said today.
Dr Blix maintained that United Nations criteria had not been met for military action in Iraq.
His comments will add more pressure on British Prime Minister Tony Blair to hold an independent judicial inquiry into the reasons for going to war.
In a separate development British Foreign Secretary Jack Straw denied a report by Dr David Kay into weapons of mass destruction (WMD) in Iraq had undermined the case for military action.
He insisted that WMD may still be discovered despite the Iraq Survey Group reporting that so far none had been found.
Dr Kay, head of the 1,200-strong CIA-led ISG which has been scouring Iraq for the last three months, declared last night: "We have not yet found stocks of weapons."
But Dr Kay's interim report to congressmen and women in Washington confirmed evidence of WMD-related programmes and indicated that Saddam Hussein had remained "firmly committed" to acquiring nuclear weapons.
In his report to the US House and Senate intelligence committees, Dr Kay said he would need another six to nine months and a further $600 million to complete his task.
"We have not yet found stocks of weapons but we are not yet at the point where we can say definitely either that such weapons stocks do not exist or that they existed before the war and our only task is to find where they have gone."
However, Dr Blix said the reasons given for war in Iraq had not met UN criteria for action. "One [reason] is that there should be a manifest threat," he told the BBC. "The intelligence was not so strong in reality that it could be said to be manifest.
"And the second one would be the imminence of it. If they can develop weapons of mass destruction in five years or 10 years, well that certainly is not imminent. So I think it probably failed, in my view, on those two counts."
Dr Blix said the UN charter allowed action in self-defence against an attack.
"It is contended now that in the day and age of biological weapons and weapons of mass destruction, one must interpret this more liberally, and that one cannot just sit and wait for them to develop their weapons fully and then attack," Dr Blix said.
"All right, if one begins to discuss that, I think one will have to put up new criteria: when would pre-emptive action really be permissible?"
But Mr Straw insisted: "The fact that they have not found weapons obviously does not mean weapons were not there. . . . There is no doubt from all the evidence that they did indeed pose a current and serious threat."
He told the BBC Radio Four's Todayprogramme: "If we had not taken military action at the time as we did, in the face of that defiance then the resolve of the international community would have died down, and then inspectors would have found it more and more difficult to do their work as they had done before."
PA